To improve high temperature stability and to provide better physical and electrical properties over amine cured epoxy resin systems, carboxylic acid anhydride curing agents have been found to be particularly useful with epoxy resins for high voltage insulation applications. With certain cycloaliphatic epoxy resins, carboxylic acid anhydrides only are used as curing agents because of the relative unreactivity of these types of epoxies with amine curing agents.
However, one serious drawback to using carboxylic acid anhydride curing agents lies in their very sluggish reactivities at temperatures close to room temperature. Usually the addition of an accelerator is required to give reasonable gel times at elevated temperatures, but at room temperature, even with high concentrations of accelerators, very slow gel times are still experienced. Under conditions of room temperature cure, it is not unusual for an epoxy-anhydride resin sample to stay tacky for several weeks before finally attaining a completely cured, tack-free condition. Considerable effort has been devoted in recent years to developing improved room temperature curing agents for epoxy-anhydride resins. This need has become even more critical with the recent natural gas shortages, which curtailed the use of heat energy to cure resins in industrial applications.
Proops, in U.S. Pat. No. 3,281,376, attempted to solve problems of room temperature cure of epoxides which could contain acid anhydride hardeners. After exploring pre-reaction of the epoxy with Lewis Acids, such as boron trifluoride and stannic chloride, and discarding those combinations because of uncontrolled exotherms, bubble formation and foaming, Proops discovered a method of pre-reacting tin organo phosphates, such as stannic tetrakis [di(propyl)phosphate], with the epoxides. This reacted epoxide could then be allowed to cure slowly at room temperature, or could be copolymerized with an organic hardener, such as an amine or a polycarboxylic acid anhydride. Smith et al., in U.S. Pat. No. 4,020,017, used minor amounts of organo-tin compounds, such as triphenyl-tin chloride, to form apparent complexes with reactive epoxide diluents, for use as additives to cycloaliphatic and glycidyl ester epoxy resins, to provide resinous electrical insulating compositions without using acid anhydrides. These compositions however, require at least 120.degree. C. curing temperatures.
Holloway et al., in U.S. Pat. No. 3,799,905, in another area, taught low temperature curable, moisture resistant dental epoxides, cured by a curing agent consisting of a BF.sub.3, SbCl.sub.5, TiCl.sub.4, SiCl.sub.4, FeCl.sub.3, AlCl.sub.3 or SnCl.sub.4 complex with a strong non-carboxylic acid, such as nitric or hydrochloric acid. The weight ratio of halide compound:non-carboxylic acid was from 50:100 to 25:100. Cure was relatively slow to allow shaping during use in restorative dentistry, Markovitz, in U.S. Pat. No. 3,728,306 and U.S. Pat. No. 3,622,524, relating to electrical grade epoxies, taught pre-reaction of organo stannoic acids and organo tin oxides respectively, with carboxylic acid anhydrides, to form reaction products that could cure epoxy resins at below 100.degree. C. The tin-anhydride reaction product, containing at least about 10 wt% organo tin compound, however, required heating at between about 75.degree. C. to 250.degree. C., from 1 to 4 hours, to dissolve the materials and form a useful, epoxy reactive, low temperature curing agent. Usually, a solid was formed which liquified at about 80.degree. C. to 160.degree. C. and which would usually require melting to react with epoxy. Again, a considerable amount of heat was required in this process.
What is needed, is a curing additive for an epoxy-carboxylic acid anhydride system that will allow complete admixing and fast cure at up to about 45.degree. C., without loss of physical, thermal or electrical properties.